This application is the U.S. national phase of International Application No. PCT/GB02/01255, filed Mar. 18, 2002, which designated the U.S., the entire content of which is hereby incorporated by reference.
This invention relates to a method and an apparatus for locating the source of an unknown signal received by a plurality of signal relays.
The invention is particularly relevant to communications using Earth-orbiting satellite relays. Interference occurring in a satellite communications channel is a serious problem that can deny use of the channel to a legitimate user. Occurrences of interference number thousands annually, and are likely to grow due to the proliferation of satellite-based services, the emergence of personal satellite communications, and the ever-increasing congestion of the geostationary arc. Interference may result from equipment failure or human error such as incorrect orientation of an antenna, but it may also represent deliberate unauthorised use of a satellite communications channel or an attempt to deny it to other users.
In IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-18, No. 2 March 1982, P C Chestnut describes the basic technique of locating an unknown signal source: it involves determining the time difference of arrival (TDOA) and/or frequency difference of arrival (FDOA) of two signals from the source relayed to receivers. The signals are relayed along two independent signal paths to a receiving station. TDOA and FDOA are also known as differential time offset (DTO) and differential frequency offset (DFO) or differential Doppler. The technique of determining DTO and DFO from two received signals is described in IEEE Trans. on Acoustics Speech and Signal Processing, Vol. ASSP-29, No. 3, June 1981 by S Stein in a paper entitled xe2x80x9cAlgorithms for Ambiguity Function Processingxe2x80x9d. The technique involves deriving the degree of correlation between the signals by multiplying them together and integrating their product. Trial relative time shifts and frequency offsets are introduced in sequence between the signals and their correlation is determined for each. The time shift and frequency offset which maximise the correlation are taken to be the required DTO and DFO, subject to correction for signal propagation delays in satellite transponders and frequency shifts in satellites and in processing.
U.S. Pat. No 5,008,679 relates to a transmitter location system incorporating two relay satellites and using both DTO and DFO measurements. The relay satellites are in geostationary or geosynchronous orbits and they relay signals along two independent signal paths to a receiving station, i.e. groundxe2x80x94satellitexe2x80x94ground paths. Each satellite accepts a signal (uplink) from the source, frequency shifts it using a turn-round oscillator and returns its frequency-shifted equivalent (downlink) to a ground receiver. The two signal path lengths are normally unequal, and this gives two signal arrival times at the receiver differing by the TDOA value. FDOA is due to relay satellite motion relative to the Earth and to one another, which Doppler shifts both downlink signal frequencies: the Doppler shifts are normally unequal because the satellites"" velocities differ, so the signals"" frequencies differ after they have passed via respective satellites. There is also a contribution to signal frequency difference from the difference between the frequencies of the two satellites"" respective frequency translation or turnround oscillators used for mixing uplink signals before retransmission for downlink. The positions and velocities of the two satellites and the receiving station""s position are known, and the locus of points of constant TDOA or FDOA is in each case a surface which intercepts the Earth""s surface to define a curve referred to as a line of position (LOP). Two measurements of TDOA or FDOA at different times, or one of each at one or more times, provides two LOPs which intersect at the position of the source to be located.
In the prior art, TDOA is also referred to as differential time offset (DTO) and FDOA as differential frequency offset (DFO) or differential Doppler shift, and the expressions DTO and DFO will be used hereinafter.
The degree of correlation is determined from what is referred to as the cross ambiguity function or CAF A(xcfx84,xcexd) defined by:                               A          ⁡                      (                          τ              ,              v                        )                          =                              ∫                                          -                T                            /              2                                      T              /              2                                ⁢                                                    s                1                *                            ⁡                              (                t                )                                      ⁢                                          s                2                            ⁡                              (                                  t                  +                  τ                                )                                      ⁢                          ⅇ                                                -                  2                                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                ⅈ                ⁢                                  xe2x80x83                                ⁢                vt                                      ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                                          (        1        )            
A(xcfx84,xcexd) is the integral of the product of two complex signals s1(t) and s2(t) after a trial time shift xcfx84 and a trial frequency shift xcexd have been introduced between them in processing after reception at the receiving station. The asterisk in s1*(t) indicates a complex conjugate. A maximum value of the modulus of A(xcfx84,xcexd), i.e. |A(xcfx84,xcexd)| is a peak in the surface |A(xcfx84,xcexd)| as a function of the two variables xcfx84 and xcexd, and the values of xcfx84 and xcexd yielding this peak are the required DTO and DFO.
Since |A(xcfx84,xcexd)| is a function of two variables xcfx84 and xcexd, it is two-dimensional and defines a surface referred to as the Ambiguity Surface: it may be calculated using a Fast Fourier Transform (FFT) technique. In one such approach a succession of lines in the Ambiguity Surface are calculated with varying xcexd (trial DFO) and respective constant values of xcfx84 (trial DTO): This effectively decomposes the surface into a series of 1-dimensional slices perpendicular to the xcfx84 axis and referred to as xe2x80x98DFO Slicesxe2x80x99. An efficient operation to compute a DFO Slice is FFT (s1*(t)s2(t+xcfx84)). Performing this computation for each practical value of xcfx84 and combining slices gives the Ambiguity Surface.
U.S. Pat. No. 6,018,312 to Haworth relates to a transmitter location system employing a reference signal passing via the same satellite relays as the unknown signal and processed in phase coherence with it. The reference signal is used to remove sources of error and operational limitations: it gives improved accuracy and extends the range of conditions over which measurements can be made. Another technique for counteracting sources of error using a broad band approach is disclosed in U.S. Pat. No. 5,594,452 to Webber et al.
International Patent Application No. GB 00/02940 relates to a modification to the technique of U.S. Pat. No. 6,018,312 to deal with the problem of time-varying DTO and DFO.
There is particular difficulty in locating a source of interference which is frequency agile, i.e. interference that is subject to changes in carrier frequency. The reason for this is as follows: the performance of the correlation process expressed by the Complex Cross Ambiguity Function depends on achieving an output signal-to-noise ratio (SNR), which is defined by                               SNR          =                      2            ⁢            BT            ⁢                          snr              1                        ⁢                                          snr                2                                            1                +                                  snr                  1                                +                                  snr                  2                                                                    ,                            (        2        )            
where B is the acquisition sample bandwidth of primary and secondary receiver channels used to receive signals from respective satellites, and T is the integration time as defined in Equation (1) for the CAF A(xcfx84,xcexd). The acquisition sample bandwidth is the bandwidth within which a signal must lie to be detectable by a receiver, and is defined by the receiver""s signal processing system. The primary channel is associated with the ground-based receiver or antenna directed at an interference-affected satellite, and the secondary channel is associated with another receiver directed at a further satellite via which an unknown transmitter causing the interference is also detectable. The terms snr1 and snr2 are respectively the input signal-to-noise ratios in the primary and secondary channels. The term 2BT is called the Processing Gain.
To achieve reliable detection of a correlation peak in the modulus |A(xcfx84,xcexd)| of the CAF A(xcfx84,xcexd), the SNR in Equation (2) should exceedxcx9c100 (i.e. 20dB): if snr1 and snr2 are fixed, this criterion defines the required Processing Gain 2BT for successful location of unknown sources.
For a fixed frequency signal from an unknown transmitter, i.e. a signal with constant carrier frequency, the maximum available Processing Gain of the relevant receiver channel is twice the channel""s integration time multiplied by its Complex Sample Rate (equal to its acquisition sample bandwidth described later in more detail). This sample bandwidth is set as close as possible to the instantaneous bandwidth of the signal to minimise the extraneous signal and noise components
An unknown transmitter may generate a signal from a non-stationary (varying) carrier frequency, in which case the associated interference with a satellite relay also varies in frequency: to acquire this interference and maintain it within a receiver channel""s acquisition sample bandwidth requires wideband sampling and therefore a higher Processing Gain than in the constant carrier frequency interference equivalent. A typical receiver channel contains analogue to digital converters (ADCs) and storage facilities, and the speed of the former and the capabilities of the latter are also limiting factors in terms of the maximum rate of change of frequency of interference that can be tolerated by the channel while still successfully carrying out geolocation. Moreover, widening the receiver channel""s acquisition sample bandwidth necessarily makes it more likely that unwanted signals will be included in the correlation operation defined in Equation (1): this adds to the noise level with respect to the interference and therefore reduces the signal to noise ratios in the primary and secondary channels; it also provides the Ambiguity Surface with additional correlation spikes not associated with the relevant unknown transmitter, and produces confusion over which spike is correct. It is therefore desirable to avoid wideband sampling and processing.
It is an object of this invention to provide an alternative method and apparatus for transmitter location.
The present invention provides a method of locating the source of an unknown signal of varying frequency received by a plurality of signal relays, the method including the steps of:
a) arranging for a plurality of receivers to receive the unknown signal via respective signal relays;
b) using a local oscillator (LO) signal to downconvert signals received by respective receivers to an intermediate frequency (IF);
c) introducing trial values of differential time offset DTO) between received signals; characterised in that the method also includes the steps of:
d) constraining the downconverted signals to lie within a prearranged bandwidth by adjusting the LO signal frequency to counteract frequency change in the unknown signal; and
e) counteracting changes in correlation phase angle of correlated signals relatively offset from one another in time and associated with differing LO frequencies by virtue of the offset and the LO signal frequency adjustment.
When the procedure of adjusting the LO signal frequency to counteract frequency change in the unknown signal was first carried out, the results of target location were much poorer than had been expected and the reason for this was very hard to find. After considerable research it was found surprisingly that adjustment of the LO signal frequency and introduction of trial values of relative time offsets introduced an error: as will be described later in more detail, it meant that for a length of time equal to the trial offset pairs of signals to be correlated were being downconverted by LO signals of different frequency. When this effect was counteracted, considerable improvement in correlation peak definition was obtained, which is the advantage of the invention.
The step of compensating for changes in correlation phase angle may comprise calculating and applying to correlation products of signals a phase correction eixcex94100, xcex94xcfx86 is a change in correlation phase angle equal to 2xcfx80xcfx84(faxe2x88x92fb), xcfx84 is a trial DTO value with which the correlated signals are relatively offset from one another and fb and fa are LO frequencies before and after adjustment respectively.
The step of downconverting signals may comprise downconverting frequency varying signals to a substantially constant frequency and band-limiting them, and further downconverting the band-limited signals for subsequent processing including digitisation and correlation.
The step of constraining the downconverted signals to lie within a prearranged bandwidth comprises identifying a frequency of maximum power in a frequency spectrum associated with received signals and determining therefrom an LO signal frequency appropriate for downconverted signals to be obtained within the prearranged bandwidth.
In another aspect, the present invention provides apparatus for locating the source of an unknown signal of varying frequency received by a plurality of signal relays, the apparatus including:
a) a plurality of receivers for receiving the unknown signal via respective signal relays;
b) a signal generator for providing a local oscillator (LO) signal to downconvert signals received by respective receivers to an intermediate frequency (IF);
c) a correlation processor for introducing trial values of differential time offset (DTO) between received signals;
characterised in that the apparatus also includes:
d) processing means for:
i) constraining the downconverted signals to lie within a prearranged bandwidth by adjusting the LO signal frequency to counteract frequency change in the unknown signal; and
ii) counteracting changes in correlation phase angle of correlated signals relatively offset from one another in time and associated with differing LO frequencies by virtue of the offset and the LO signal frequency adjustment.
The processing means may be arranged to calculate and apply to correlation products of signals a phase correction eixcex94xcfx86,xcex94xcfx86 is a change in correlation phase angle equal to 2xcfx80xcfx84(faxe2x88x92fb), xcfx84 is a trial DTO value with which the correlated signals are relatively offset from one another and fb and fa are LO frequencies before and after adjustment respectively. It may also be arranged to downconvert frequency varying signals to a substantially constant frequency and band-limit them, and further downconvert the band-limited signals for subsequent processing including digitisation and correlation. It may further be arranged to constrain the downconverted signals to lie within a prearranged bandwidth by identifying a frequency of maximum power in a frequency spectrum associated with received signals and determining therefrom an LO signal frequency appropriate for downconverted signals to be obtained within the prearranged bandwidth.